A lithium secondary battery has a structure in which an assembly is composed of a cathode and an anode including an electrode active material coated on a current collector, the electrode active material being capable of intercalating and deintercalating lithium ions, with a porous separator interposed between the cathode and the anode to electrically separate them, and the assembly is filled with an organic electrolyte solution or polymer electrolyte solution containing a lithium salt.
As a cathode active material, lithium metal oxide having a high average voltage (for example, LiCoO2, LiNiO2, LiNixCoyAlzO2, LiNixCoyMnzO2, and LiMn2O4) is mainly used, and as an anode active material, a carbon material or metal or non-metal oxide having a low average potential is primarily used.
In the case where a cathode active material is used without a post-processing process, that is, in the case where a commercially available active material produced without surface treatment and treatment processes is used, a transition metal-deficient layer is formed on the surface of the active material due to a decomposition reaction between an electrolyte solution and a metal, and the transition metal-deficient layer or a resistor film on the surface of the active material impedes movement of lithium ions and electrons, affecting high efficient discharge, and due to a side reaction with an electrolyte solution, gas is generated inside a battery and a metal is released, as a result, deterioration in cycling characteristics occurs due to a structural change. Also, as a battery operates abnormally, internal temperature of the battery increases and oxygen is generated, which features poor safety, inducing a thermal runaway phenomenon.
In the case where a carbon-based anode active material is used, an irreversible capacity exhibiting in lithium ions intercalated within a layered structure during initial charging and discharging is 5 to 25%, and this irreversible capacity promotes consumption of lithium ions and prevents complete charging or discharging of a minimum of one active material or more, which features a reduced energy density of a battery.
Also, a decomposition reaction of an electrolyte solution on the surface of an active material causes the formation of a passivating layer or a solid electrolyte interface on the surface of the active material, and in this instance, when the passivating layer is formed non-uniformly or thickly, an increase in resistance causes deterioration in high rate characteristics. Also, as a lithium compound is generated on the surface of an anode, a capacity reduction and output characteristics degradation results from lithium loss, and in the long run, deterioration in cycling characteristics occurs.